Most homeowners will be familiar with circuit breakers as typically employed in home wiring. Home circuit breaker assemblies typically provide two main breakers connecting the power lines to two bus bars, connected to line power, and extending the length of a breaker housing. Individual circuit breakers can then be snap-fit into place, such that one of the contacts controlled by the mechanism of the breaker is connected to the appropriate bus bar. The switch mechanism of the breaker can then be thrown by hand to make or break the contact between the bus bar and the load terminal to which the load is connected, and of course, the switch mechanism disconnects automatically upon detection of an overload. As noted, breakers of relatively low capacity, up to on the order of fifty amperes, are connected by hand to the bus bars assembly.
The functions provided by circuit breakers typically employed at substations and the like, to connect transcontinental power lines and the like with factories, housing developments, and the like, are generally similar, but have much higher voltage and current ratings.
The art is well aware that it is important to provide mechanisms for insuring that such breakers cannot be physically connected between the line and load when the breaker is in the contacts-closed position, i.e., the breaker must be maintained "trip-free" during the installation process. Moreover, the art shows means for prevention of removal of an installed breaker while in the contacts-closed position; this prevents a breaker from being physically moved out of a circuit including an active load, which can cause damage.
Further, the art teaches that it is desirable to provide a keyed locking mechanism for securing breakers in various positions, including not only the active or "Connect" position in which the breaker is connected to the line and load connections, but also in the "Test" position, wherein the contacts can be closed for testing, and the "Disconnect" position, in which the breaker is placed when initially installed, and from which the breaker can be removed. For example, locking the breaker in the Disconnect position allows maintenance operations to be carried out "downstream" of the breaker.
Since high capacity breakers are usually heavy, mechanical mechanisms are provided for "racking", i.e., physically moving, a breaker initially placed in the Disconnect position through the Test and into the Connect position. Typically, interlocks are used to insure both that the breaker cannot be racked into the Connect position while its contacts are closed (for the reasons mentioned above), and also to ensure that only breakers having capacity appropriate for the associated circuits can be racked into the connected position. The racking mechanism is mounted in a "cubicle", for moving the breaker toward or away from line and load terminals disposed within the cubicle.
In some circumstances it may be desirable to fit a "dummy" breaker, that is, a device providing permanently closed connections between the line and load terminals as required, and not including the overload disengagement mechanism and other safety features of an operable breaker, in circuit breaker receiving cubicles using the same racking mechanism required to accept a suitable operable circuit breaker. Circuit protection is then provided by "downstream" breakers.
The art includes mechanisms providing a keyed locking mechanism operable to prevent the dummy breaker from being withdrawn without the key being present, reducing the chance that a dummy breaker may be withdrawn mistakenly. As a rule, it is desirable to be able to lock a dummy breaker in the Connect position, but it is not desirable to be able to thus lock an operable breaker. Hence, an interlock device must be provided to distinguish between dummy and operable breakers and allow locking only dummy breakers in the Connect position.
Prior art mechanisms for providing the various interlock and features discussed above, and others known to those in the art, have tended to be undesirably complicated and expensive. In particular, interlocked lockable breaker racking mechanisms provided prior to the present invention have required two separate keys for locking the breaker type detection means (for example, to ensure that only dummy breakers can be locked in the Connect position), and for locking the racking device in a desired position. This involves excessive complexity.
The prior art is generally summarized by U.S. Pat. No. 5,278,722 to Peruso, which is commonly assigned with the present invention to Siemens Energy and Automation, Inc. and by the assignee's publications Nos. SG3258-01 and SG3268-03 describing commercially available products. The Peruso patent teaches a racking mechanism for switchgear, including operable or dummy circuit breakers. As described therein, the racking mechanism includes a screw block translated by rotation of a lead screw device to rack a circuit breaker into engagement with the appropriate line and load contacts. The lead screw assembly is operable by a crank having a shield arranged such that the breaker can only be moved between the Disconnect and Connect positions when the appropriate crank is inserted into the racking mechanism. Matching apertures in sliding and stationary cam assemblies are provided to receive the hasp of a padlock for locking the breaker assembly in any of several desired positions. The Peruso patent teaches trip-free interlock mechanisms, for example, precluding racking a breaker into the Connect position when its contacts are closed.
As will be apparent from review of the present application with respect to Peruso, the present application describes numerous improvements on the basic Peruso mechanism. Where not discussed specifically, details of the racking assembly according to the invention can be identical to those shown by Peruso and exemplified by commercial products forming part of the prior art applicable to this invention.
In particular, it was desired to provide an interlock mechanism which allows only breakers of the correct rating and/or type (i.e., operable or dummy breakers) to be locked in the Connect position, while also simplifying the locking mechanism provided to secure the breaker in one or more desired conditions or positions. The result would be a reduction in cost and an improvement in functionality.